1. Field of the Invention
The invention relates to a method and apparatus to grow crystals for electronics and photonics applications. More particularly the present invention relates to a method and apparatus employing the Czochralski (“CZ”) single-crystal growth process, wherein multiple crystal boules may be grown and cooled during a single heating cycle of the crystal growth furnace.
2. Description of the Prior Art
In the known Czochralski (“CZ”) single-crystal growth process a sealed furnace structure having a noble metal crucible containing a crystal forming granular aggregate is heated in an inert atmosphere, forming a melt. A crystal growth seed rod is placed in contact with the melt and withdrawn at a speed that promotes growth of a single crystal boule. After the crystal boule is grown the furnace is cooled slowly in order to minimize risk of boule cracking or creation of other cooling induced imperfections. The cooled furnace chamber is opened, so that the boule may be removed for further processing. After cooling and boule removal, the furnace chamber and growth components are readied for another crystal growth cycle. Due to cooling constraints, considerable time (often many days) is required to complete a complete crystal growth cycle and ready the growth furnace for the next growth cycle.
In order to reduce cycle time between single crystal boule growth cycles, in the past it has been suggested to grow multiple crystal boules in a single crystal production cycle. One suggested solution has been to grow single boules serially in a single crucible and transferring grown crystal boules to a holding area in the furnace. Another suggested serial processing solution was to create a two-part furnace having the crucible in the first part and a removable growth rod/boule extraction tower removable from the first part. Upon completion of a boule growth, the tower section would be removed (potentially wasting heat as the furnace is opened and discharging inert gas normally occupying the furnace chamber when practicing the CZ process) and replenished with a new tower section. In either of these serial processing solutions, when the serial growth run was completed the furnace was cooled and multiple completed boules extracted. The furnace would then be prepared for another growth cycle.
Another suggested batch processing solution has been to grow simultaneously multiple crystals in parallel with multiple crucibles in a single furnace. Again, upon completion of the parallel growth cycle the furnace would be cooled, the multiple boules extracted and the furnace serviced for commencement of another production cycle.
In preparation for a subsequent growth cycle the furnace and components are serviced and repaired, as is often necessary, due to the high-temperature thermal stresses on the components. Individual thermal stress events which are exacerbated by thermal cycling from initial cold state to heated state and back to cold state. Crucible assembly repair and servicing is critical because it is subject to very high thermal stress, and thus is prone to warping and cracking. Additionally after a crystal growth cycle a cooled crucible contains re-solidified residual melt and slag that is difficult and time consuming to remove without damaging the crucible. When the noble metal crucible can no longer be repaired due to cracks and warpage, it must be scrapped and recycled due to the value of its material.
The same equipment servicing challenges exist for a serial or parallel multiple boule processing production cycle as does for a single boule processing cycle furnace. Additional new challenges for multiple boule processing cycles include: re-charging spent melt, if multiple boules are to be extracted from a single crucible; and waste slag removal from the crucible as more melt is added to a growth crucible. As slag builds in a growth crucible, less crucible volume is available for new melt.
A past solution for providing recharge melt for crystal formation crucibles has been to melt solid aggregate in a first melting crucible and then feed the melt to a downstream crystal formation crucible. Two common structural geometries for the dual melting/crystal formation crucibles have included coaxially nested crucibles or inclusion of siphon/gravity feed tubes from the melt crucible to the crystal formation crucible.
While past continuous crystal growth systems addressed crystal melt replenishment, they did not propose solutions for crucible slag accumulation resulting from melt replenishment. As is known by those skilled in the art, variations in slag concentration can negatively impact uniformity of dopant distribution within a melt at the crystal-melt interface. Deviations in dopant distribution in a single boule (e.g. variations at the top of the boule vs. the bottom of the boule) or in a series of boules will negatively impact uniformity of boule optical and scintillation properties.
There are needs in the crystal growth field to: (i) reduce boule fabrication cycle time; (ii) reduce heating energy costs associated with operation of a CZ crystal formation furnace; (iii) reduce service and maintenance costs associated with operation of CZ crystal formation furnaces and (iv) achieve boule uniformity of optical and scintillation properties within a boule or series of fabricated boules.